An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
A typical LCD includes an LCD panel, a plurality of backlights for illuminating the LCD panel, an inverter circuit for driving the backlights, and a backlight control circuit. The backlight control circuit generally includes a pulse width modulation integrated circuit (PWM IC) for driving the inverter circuit, and a backlight protection circuit for shutting down the PWM IC when any one of the backlights has an open circuit or a short circuit connecting to ground.
FIG. 3 is an abbreviated diagram of a typical backlight control circuit used in an LCD. The backlight control circuit 100 includes four load circuits 110, a PWM IC 150, and a backlight protection circuit (not labeled). The backlight protection circuit includes a first transistor 171, a current limiting resistor 172, and an input circuit 130.
Each load circuit 110 includes a backlight 111 and a backlight inspecting circuit 113 connected in series between a power supply (not shown) and ground. The backlight inspecting circuit 113 includes an output end 112. The output end 112 provides a high voltage when the corresponding backlight 111 works. The output end 112 provides a low voltage when the corresponding backlight 111 has an open circuit or a short circuit connecting to ground.
The PWM IC 150 includes a current sampling pin 151. The PWM IC 150 stops working if the current sampling pin 151 has a low voltage.
The first transistor 171 includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” is connected to ground. The drain electrode “D” is connected to the current sampling pin 151 of the PWM IC 150. The gate electrode “G” is connected to a power supply via the current limiting resistor 172. The power supply is provided by a power pin (not labeled) of the PWM IC 150.
The input circuit 130 includes four diodes 131, four resistors 132, four capacitors 135, a second transistor 1332, a third transistor 1333, a fourth transistor 1334, and a fifth transistor 1335. Each transistor 1332, 1333, 1334, 1335 includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The drain electrode “D” of the second transistor 1332 is connected to the gate electrode “G” of the first transistor 171. The drain electrode “D” of the third transistor 1333 is connected to the source electrode “S” of the second transistor 1332. The drain electrode “D” of the fourth transistor 1334 is connected to the source electrode “S” of the third transistor 1333. The drain electrode “D” of the fifth transistor 1335 is connected to the source electrode “S” of the fourth transistor 1334. The source electrode “S” of the fifth transistor 1335 is connected to ground. The gate electrodes “G” of the second, third, fourth, and fifth transistors 1332, 1333, 1334, 1335 are connected to negative terminals of the four diodes 131, respectively. Positive terminals of the four diodes 131 are respectively connected to the output ends 112 of the backlight inspecting circuits 113. Each of the gate electrodes “G” of the second, third, fourth, and fifth transistors 1332, 1333, 1334, 1335 is connected to ground via the corresponding resistor 132, and is connected to ground via the corresponding capacitor 135.
The first transistor 171, the second transistor 1332, the third transistor 1333, the fourth transistor 1334 and the fifth transistor 1335 are negative-channel metal oxide semiconductor (NMOS) type transistors.
The operation of the backlight control circuit 100 is as follows. When all the backlights 111 work normally, each of the output ends 112 provides a high voltage to the corresponding gate electrode “G” of the second, third, fourth, and fifth transistor 1332, 1333, 1334, 1335 via the corresponding diode 131. Then the second, third, fourth, and fifth transistors 1332, 1333, 1334, 1335 are switched to an activated state, and the gate electrode “G” of the first transistor 171 is connected to ground via the activated second, third, fourth, and fifth transistors 1332, 1333, 1334, 1335. Thus the first transistor 171 is turned off, and the current sampling pin 151 of the PWM IC 150 maintains an original working voltage.
When any one of the backlights 111 has an open circuit or has a short circuit connecting to ground, the corresponding output end 112 provides a low voltage to the corresponding gate electrode “G” of the second, third, fourth, and fifth transistors 1332, 1333, 1334, 1335 via the corresponding diode 131. Then the corresponding second, third, fourth, or fifth transistor 1332, 1333, 1334, 1335 is turned off, so that the gate electrode “G” of the first transistor 171 is charged to a high voltage by the power supply via the current limiting resistor 172. Thus the first transistor 171 is switched to an activated state, and the current sampling pin 151 of the PWM IC 150 is connected to ground via the activated first transistor 171. Consequently, the current sampling pin 151 of the PWM IC 150 is charged to a low voltage, and the PWM IC 150 stops working.
The backlight control circuit 100 includes the five transistors 171, 1332, 1333, 1334, 1335 needed to carry out the function of protecting the backlights 111. Further, the number of transistors needed increases with the number of backlights 111 used in the LCD. Consequently, the cost of the backlight control circuit 100 is high, particularly in the case where the number of backlights 111 is large.
It is desired to provide a backlight control circuit used typically in an LCD which overcomes the above-described deficiency.